1. Field of the Invention
The present invention is related to pulse stretchers, and more particularly, to pulse stretchers for use in lithographic systems.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or glass substrate.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithography each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
A difficulty in designing optical components used in microlithography is caused by the high power density involved during exposure. High-powered lasers are often used as a source of illumination in lithographic systems. These lasers are usually pulsed lasers, which generate relatively short high intensity pulses. The high intensity pulses, over time, damage optical components, such as lenses used in the projection optics, requiring frequent replacement of these optical components. To reduce the effect of the high power density, pulse stretchers are often used in lithographic equipment. A pulse stretcher is a passive optical component that uses an arrangement of beam splitters to extend the duration of the pulse. Pulse stretchers can sometimes be cascaded, to multiply the effect of a single pulse stretcher. However, the volume of space available for the projection optical system with the lithographic tool is limited. Consequently, the volume of space available for the pulse stretcher is also limited, often precluding the use of a large number of cascaded pulse stretchers.
Thus, the purpose of the pulse stretcher is to maintain the energy of the pulse generally constant, while reducing peak intensities by spreading out the pulse in time.
The conventional systems use one or two beam splitters and a series of mirrors that split a single laser pulse into two strong pulses and a series of weaker pulses. Several pulse stretching devices can be used in tandem to produce more pulses over a longer time. See, e.g., Japanese patent JP6-214187 and U.S. Pat. No. 6,389,045.
The lithography industry is trying to achieve very large pulse stretching in order to extend the lifetime of the optics. With the conventional designs, the delay is so long that the delay path must be folded many times to fit in a reasonable volume. This creates alignment difficulties. Alternatively, many shorter delays can be applied in tandem, but this also requires many mirrors, which are difficult to align. Also, the limited number of “copies” of the pulse achievable with practical pulse stretcher designs leaves sharp peaks in the pulse profile. Further reduction of the peak intensities could be achieved if many overlapping pulses were produced, resulting in smoothing.
Accordingly, what is needed is a pulse stretcher that is compact and permits a significant stretching of the pulse from a lithographic illumination source.